It is known for the purposes of exhaust gas purification, in particular in motor vehicle diesel engines, to use externally supplied reducing agents with which the nitrogen oxide content in the exhaust gases is reduced. The reducing agent is generally injected into the exhaust gas stream by an injection device. An “SCR” catalytic converter located downstream of the injection device then carries out the actual reaction. SCR is the name used to describe the technique of Selective Catalytic Reduction (SCR) of nitrogen oxides in exhaust/waste gases from combustion plants, refuse incinerators, gas turbines, industrial plants and motor vehicle combustion engines, in particular diesel engines. The chemical reaction on the SCR catalytic converter is selective, i.e. nitrogen oxides (NO, NO2) are preferentially reduced, while unwanted secondary reactions (such as for example the oxidation of sulfur dioxide to sulfur trioxide) are very largely suppressed. SCR catalytic converters are often used in combination with soot particle filters and oxidation catalytic converters.
A reducing agent is required for the above-stated reduction reaction, ammonia (NH3) typically being used for this purpose. The required ammonia is here not generally used directly, i.e. in pure form, but instead in the form of an aqueous urea solution, which is known industrially by the standard name AdBlue®. This substance will hereinafter be denoted “reducing agent solution.”
The reason why the required ammonia is not carried in pure form is due to the hazardous nature of this substance. Ammonia has a corrosive action on the skin and mucous membranes (in particular also on the eyes), it additionally forms an explosive mixture in air and even in low concentrations has an unpleasantly pungent odor.
The composition of AdBlue® is governed by DIN 70070; the urea content in the aqueous solution specified in this standard is 32.5%±0.7%.
On injection into the hot exhaust gas stream, the above-stated urea solution undergoes a decomposition reaction forming ammonia and carbon dioxide. The ammonia produced in this manner is then available to the SCR catalytic converter which is arranged downstream. When ammonia reacts with the nitrogen oxides in the exhaust gas, a comproportionation reaction takes place, giving rise to water (H2O) and nitrogen (N2).
Selective catalytic reduction removes a large proportion of the nitrogen oxides from the exhaust gas. In contrast with the diesel particulate filter (DPF) or lean NOx traps (LNTs), this pollutant reduction does not entail any additional fuel consumption, as, unlike the above-stated catalytic converters, an SCR catalytic converter does not require a temporary deviation from ideal combustion conditions when in operation.
The quantity of urea to be injected depends on the nitrogen oxide emission from the engine and thus on the engine's instantaneous rotational speed and torque. Consumption of urea-water solution includes, depending on the engine's crude emission level, approximately 2 to 8% of the diesel fuel used.
Dispensing of the urea is here adjusted to the NOx in the exhaust gas mass flow by means of a “feed ratio”. If too much urea is apportioned, the ammonia formed therefrom can no longer be consumed by reaction with NOx. This dispensing error may result in ammonia getting into the environment. Since ammonia can be perceived at even very low concentrations, this results in an odor nuisance. This phenomenon is avoided by using specific downstream catalytic converters which process the excess ammonia by reaction, but this entails additional structural expense.
Under-dispensing urea leads to unsatisfactory exhaust gas purification and is therefore likewise undesirable.
When dispensing the reducing agent solution into the exhaust gas stream according to the respectively calculated feed ratio, it may be assumed that the urea content of the reducing agent solution corresponds to the respective predetermined value (in particular 32.5% for AdBlue®).
It has, however, been found in practice that it cannot be assumed that this concentration is always and in every case exactly complied with. In particular in the case of excessively long or improper storage, the concentration of urea may fall due to decomposition of urea into ammonium hydroxide and carbon dioxide.
If the actual urea content deviates from the standard value, this results in incorrect dispensing of the reducing agent solution with the above-described negative consequences which especially in the case of over-dispensing have a particularly negative impact. However, under-dispensing should also be avoided, as in this case the SCR catalytic converter cannot operate properly.
Even the tolerance of ±0.7% with regard to urea content which is admissible according to DIN 70070 means that, in order to avoid any over-dispensing which has a particularly negative impact when metering the reducing agent solution, there is a tendency to assume the upper tolerance limit (unless any additional structural measures are provided for breaking down excess ammonia). Falling below these tolerances therefore tends to have a still greater effect.
The inventors herein have recognized the above issues and provide a method for monitoring a reducing agent solution composition in an exhaust gas system of an internal combustion engine with metered injection of the reducing agent solution upstream of an SCR catalytic converter. In one embodiment, the method comprises metering injection of the reducing agent solution into the exhaust gas stream, determining a measured variable of a change in water content in the exhaust gas stream in response to the injected reducing agent solution, and determining at least one indicator value of a composition of the reducing agent solution at least based on the determined measured variable.
In this way, the composition of the reducing agent solution (e.g., the urea concentration) may be determined based on a change in water content of the exhaust stream following injection of the reducing agent solution. The reducing agent solution may be assumed to have a given urea concentration that results in an expected change in water content following injection of the reducing agent solution. If the change in water content is different than expected, the assumed given urea concentration may be adjusted, and the amount of reducing agent solution metered to the exhaust gas stream may be adjusted to provide a constant, known concentration of urea in the exhaust gas stream, even if the composition of the reducing agent solution changes over time.
By doing so, a desired amount of reducing agent solution may be injected to the exhaust gas stream upstream of the SCR device. Thus, the production of excess ammonia, which may be harmful to the environment if released, may be avoided. Further, the production of too little ammonia, which may compromise conversion of NOx species in the SCR device, may also be avoided.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.